What does "has been on message" mean? Does it mean "has been texting only without speaking"?

“Trump has been on message about Mueller since Ty Cobb came in as his lawyer” in July, Mr. Wisenberg said. “It’s pretty clear when Ty Cobb came in, he tightened up the ship and had a talk with Trump and must have said: ‘You’re O.K. on collusion. Stop attacking Mueller directly.’”_________________Hiromichi

What does "has been on message" mean? Does it mean "has been texting only without speaking"?

“Trump has been on message about Mueller since Ty Cobb came in as his lawyer” in July, Mr. Wisenberg said. “It’s pretty clear when Ty Cobb came in, he tightened up the ship and had a talk with Trump and must have said: ‘You’re O.K. on collusion. Stop attacking Mueller directly.’”

No. Trump often confuses things, and says one thing and then another contradictory thing. However, his lawyer must have told him to stop attacking Mueller directly, and as a result, he has been on message; that is, he has kept the same message about it.